1. Field of the Invention
The present invention relates to a steam valve used in a steam turbine plant or like, and more particularly, to a steam valve having an improved strainer accommodated in a valve casing and adapted to remove impurities, or foreign objects or materials contained in steam.
2. Related Art
Generally, steam-turbine plants have many steam valves having different diameters in accordance with their functions or applications. In these steam valves, a main-steam stop valve and a steam-regulation valve, for example, both of which are provided between a steam turbine and a steam generator, such as a boiler, are inevitably very large pressure-resistant valves since both valves are exposed to ultra-high-pressure (16.6 MPa to 24.1 MPa) and very-high-temperature (538° C. to 566° C.) steam.
Such a very large main-steam stop valve or steam-regulation valve, which is exposed to ultra-high-pressure and very-high-temperature steam, has a strainer accommodated in a casing. This strainer prevents welding scraps generated or tools left during construction from colliding with a valve body when a steam turbine plant is subjected to a ventilation test in a trial run before normal operation to thereby prevent accidents from causing.
In a steam valve accommodating a strainer having such a function, as shown in FIG. 9, main steam guided from a boiler through a steam inlet 1 strikes a closed area 3, which faces the steam inlet 1, of a strainer 2 and then moves in the circumferential direction of the strainer 2. The moved main steam flows into the strainer 2 through holes or perforations 4 of the strainer 2 to be supplied to a steam turbine through a steam outlet 5.
This steam valve structure prevents foreign objects contained in steam that enters through the steam inlet 1 during a trial run from coming into direct contact with a screen 7 of the strainer 2. Immediately after a trial run starts, a relatively large foreign object is often floated. If such a foreign object struck the screen 7 directly, the foreign object would break the screen 7 and enter the steam valve. In order to prevent this entry, there is adopted a structure in which a floated foreign object firstly strikes the closed area 3 of the strainer to reduce its speed so that the screen 7 reliably captures the foreign object.
As shown in FIGS. 9 and 10, relatively fine impurities generated in a boiler, such as oxide scale, are removed by the use of the screen 7 circumferentially fixed in a body 6 of the strainer 2.
The screen 7 has a three-layered (triple wall) structure consisting of a permanent screen unit 8 circumferentially mounted on a part of the body 6 of the strainer 2, a temporary screen unit 9 arranged outside the permanent screen unit 8, and a protective screen unit 10 arranged further outside the temporary screen unit 9.
In this three-layered screen 7, the permanent screen unit 8 is composed of a relatively-coarse-mesh wire net, the temporary screen unit 9 is composed of a relatively-fine-mesh wire net, and the protective screen unit 10 is composed of a relatively-coarse-mesh wire net. As shown in FIG. 11, these screen units are fixed to and supported on the body 5 of the strainer 2 by means rivets 11, for example.
In a recent large-capacity steam-turbine plant, the permanent screen unit 8 is composed of a coarse-mesh wire net to remove impurities, such as oxide scale produced in a boiler, during normal (steady) operation of the plant, the temporary screen unit 9 has a fine-mesh wire net structure to remove impurities during a trial run (test operation), and the protective screen unit 10 has a coarse-mesh wire net structure to protect the temporary screen unit 9.
The mesh size of each wire net is determined in consideration of strength, pressure loss of steam, and the degree of removal of impurities.
In the three-layered screen 7, the temporary screen unit 9 and the protective screen unit 10 are used during a trial run, and after the trial run, both screen units are demounted or removed. For the normal operation, the permanent screen unit 8 is again fixed to and supported on the strainer body 6 by the rivets 11.
Japanese Unexamined Patent Publication (KOKAI) No. HEI 10-8913 discloses a steam valve, such as an intercept valve, used in a steam turbine plant, having a strainer.
In such a steam valve having a strainer to which a three-layered screen is circumferentially mounted, impurities or foreign objects contained in steam are reliably prevented from entering the strainer, and therefore, a steam turbine can operate safely in a stable condition. However, thermal shock caused by ultra-high pressure and very-high temperature steam has a direct impact on a wire-net screen. Therefore, with years of use, the wire net is expanded, contracted, or loosened, producing vibration or noise, and finally, the wire net will be broken, providing a significant matter of defective.
Nowadays, in order to improve thermal efficiency in a plant, a steam temperature in a steam turbine is prone to be high. This results in a decrease in the allowable stress of a screen. Although such defect may be eliminated by strengthening the screen, if the diameter of each wire of the wire net is increased, the mesh size is inevitably large and fine or minute impurities are thus not removed. In other words, an improvement in thermal efficiency runs counter to an increase in strength.